Conventional modulation methods in digital wireless communication include frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). While FSK is a nonlinear modulation method, it is remarkably power-efficient, being effective in miniaturization and reduction of power consumption, and has been used in wireless call systems and the like.
However, with the increase in rate and capacity of recent wireless communications, linear modulation schemes such as PSK and QAM are being widely used in many wireless communication systems due to their excellent spectral efficiency. QAM is effective in increasing spectral efficiency, and can transmit a lot of data in a narrow band since it superimposes signal data to both amplitude and phase. It is therefore widely used in high-rate transmission modes and the like of fixed micro relay systems and wireless local area network (LAN) systems. However, QAM has low power efficiency, and requires a high signal-to-noise power ratio as multi-valuing increases.
PSK superimposes signal data only to the phase, and its frequency use efficiency is inferior to multi-level QAM schemes such as 64QAM and 256QAM. However, it is superior to FSK and QAM in regard to having excellent bit error rate characteristics, even in a low signal-to-noise ratio. This is effective when performing wireless transmission over long distances while suppressing transmission power; in particular, binary PSK (BPSK) and quadrature PSK (QPSK) are one of the most widely used digital wireless modulation methods in satellite communication systems, mobile telephone systems, low-rate transmission mode of wireless LAN, etc.
When considering effective use in the frequency band, the issue must consider the time axis and not only the frequency axis. Wireless communication systems today mainly use time division multiplexing (TDM) for transmitting multiplexed packets (or bursts) on the time axis. The TDM transmission method can be described as a multiplex method suitable for flexibly modifying and allocating lengths and numbers of packets on the time axis.
FIG. 26 is a block diagram of a conventional wireless transmitting apparatus used in TDM transmission schemes and the like. The conventional wireless transmitting apparatus includes a symbol generating circuit 101 that converts a transmission data bit stream S100 to an information symbol sequence, a preamble generating circuit 102 that generates a preamble (training signal) S102 and includes a carrier recovery signal generating circuit, a clock recovery signal generating circuit, and a frame synchronization signal generating circuit, a multiplexing circuit 103 that generates a transmission burst signal S102 by multiplexing the preamble S102 and the data symbol sequence S101, and a digital-to-analog conversion circuit 104 that converts the transmission burst signal S103 from digital to analog. FIG. 28 is an example of the frame configuration of the transmission burst generated by a conventional transmitting apparatus.
FIG. 27 is a block diagram of a conventional wireless receiving apparatus. A sequential demodulation wireless receiving apparatus using conventional synchronization detection includes an A/D conversion circuit 201 performs analog-digital (A/D) conversion of an analog received burst signal S200 and converts it to a digital received burst signal, a carrier recovery circuit 202 that extracts a carrier recovery signal portion from a received burst signal S201 and performs carrier recovery, a symbol timing synchronization circuit 203 that, after carrier recovery, extracts a symbol timing recovery signal portion and performs symbol timing recovery, a channel estimation circuit 204 that estimates propagation channel distortion based on a detected symbol timing, a channel distortion correction circuit 205 that corrects channel distortion using information related to channel distortion, a symbol decision circuit 206 that, after channel distortion correction, identifies a received data symbol and converts it to a received data bit stream, a frame detection circuit 207 that, after symbol decision, extracts a frame synchronization signal and performs frame detection, and a frame synchronization circuit 208 that performs frame synchronization using a detected frame position.
To extract burst synchronization signals such as symbol timing and carrier frequency and the like in wireless packet (burst) transmission, a training signal sequence represented by a preamble signal and a pilot signal is provided in the wireless burst. These training signals are used in compensating channel distortion that fluctuates within the burst. Since preamble signals and pilot signals are redundant signal that do not carry data themselves, they reduce transmission efficiency (frame efficiency). This reduction in transmission efficiency (frame efficiency) is particularly noticeable when the packet length is short.
There has been proposed a block demodulation method of suppressing reduction in the transmission efficiency (frame efficiency) by storing received burst signals that were quasi-coherently detected by a local oscillator of the receiver itself in a temporary memory, and performing burst synchronization and demodulation after extracting a signal for synchronization from the stored received burst signals (see non-patent literature 1). The block demodulation method makes it possible to temporarily store and repeatedly read all received burst signals, and to perform processes of symbol timing synchronization, carrier frequency estimation, channel estimation, etc. The training signal can therefore be shortened, and deterioration in transmission efficiency (frame efficiency) can be suppressed.    [Non-Patent Literature] Junji Namiki, ‘Block Demodulation for Short Radio Packet’, in The Transactions of the Institute of Electronics and Communication Engineers of Japan, 1984, vol. J67-B, No. 1, pp. 54-61.